the best way to explain it is find the rhythm of the sitting trot before you actually sit, so as you come out of the canter you will do a half halt with your seat, as soon as you feel your horse start to drop into the trot as it should start to swing your hips/pelvis in the sitting trot motion so that your pelvis starts the trot instead of trying to catch up to the motion. that is why you are bouncing bc you are trying to catch up, and remember to always push the horse into the trot from the canter or any gait so you have a nice fluid forward motion to carry on with and keep the horses back lifted.
i hope this helps, if not i can try and explain it a little more. Im working on this very same thing with my student right now.
"The horse you get off is not the same as the horse you got on; it is your job as a rider to ensure that as often as possible the change is for the better."